How does RIPv2 knows which subnet mask to be advertised

nyx01xnyx

hi guys,

How does RIPv2 knows which subnet mask to be advertised? it's based on the route installed in the routing table or the mask configured on the outgoing interface?

 

thanks!

qqabdal

Hi,

With or without auto-summary?

nyx01xnyx

Without auto-su

bjorn.timmer

As you can see in my capture the subnet mask is advertised in RIPv2.

qqabdal

Without auto-summary it is based on the mask configured on the interface where the IP is configured

nyx01xnyx

yes, i know RIPv2 routing update will includes the subnet mask.

i want to know how does RIPv2 knows about it

nyx01xnyx

hi, i did some testing. Let's say if you configured loopback0 as /32 but outgoing interface is /24. the ripv2 peer will learn /32 instead of /24. So, i  dont think the subnet mask is coming from the interface.

Brian McGahan

It's included in the RIPv2 update field. RIPv1 only includes the prefix in the update. RIPv2 includes the prefix and length. In RIPv1 the length is inferred based on the incoming interface if the prefix is a subnet of the classful network. This is why RIPv1 does not support VLSM but RIPv2 does. 

Brian McGahan, 4 x CCIE #8593 (R&S/SP/SC/DC), CCDE #2013::13
[email protected]
 
Internetwork Expert, Inc.
http://www.INE.com

On Oct 31, 2014, at 12:54 PM, "nyx01xnyx" <[email protected]> wrote:

hi, i did some testing. Let's say if you configured loopback0 as /32 but outgoing interface is /24. the ripv2 peer will learn /32 instead of /24. So, i  dont think the subnet mask is coming from the interface.

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MartinLosik

mask taken from interface loopback, so it is /32 for loopback network. the other  mask /24 taken from its interface.

so the update will include 2 networks: one with /32, and /24

nyx01xnyx

hi brian,

thanks for your reply. But, how does RIPv2 update knows the subnet mask?

Where does the subnet mask come from? Is it based on the mask which configured on the interface or the based on the route installed in the routing table?

nyx01xnyx

hi martin,

if the route is from EIGRP /25 and redistributed into RIPv2. I think the peer will learn the route with /25.

cristian.matei

Hi,

   Directly connected routes in the routing table get the mask from the interface, thus RIP gets it from the interface as well.

Regards,

Cristian.

Murad

How did you do the tests?

I dont think IOS will allow you to configure loopback with /32 and another interface with /24 untill you configure the subnet mask on the other interface to exclude the IP for the loopback.

In my opinion, RIPv2 first updates the RIP DB according to the directly connected interfaces along the subnet mask and then populates the routing table. In the update to others, it should send the routes along with subnet mask from the RIP DB, not from the routing table.

Plz correct me if i am wrong.

HTH

 

hi, i did some testing. Let's say if you configured loopback0 as /32 but outgoing interface is /24. the ripv2 peer will learn /32 instead of /24. So, i  dont think the subnet mask is coming from the interface.

 

nyx01xnyx

Hi Murad,

I tested again, you can config /32 on your loopback interface.

I think you are right. RIP will use the direct connected interface subnet mask.

If the route is redisributed from other protocols, RIP won't change the subnet mask.